This invention relates to thermoplastic olefins and to methods of using them to form shaped articles.
Plastics are potentially well suited for manufacture of large surface, deep parts such as automotive dash boards and bumper fascia or appliance housings such as refrigerator chests. The manufacturing method of choice for such parts is thermoforming. Thermoforming is conducted on an already fabricated product, a solid phase polymer sheet. Thermoforming is a deformation process of heating a polymer sheet until it is softened; stretching the sheet over a solid, cool mold having the desired part shape; holding the hot and flexible sheet and its edges against the contours of the mold; and allowing the sheet to cool until the sheet has rigidified so that it retains the shape and detail of the mold. Then the formed part is trimmed from its web. The trim, if substantial, must be recovered, reground and re-processed. Thermoformed products must bear the cost of the fabrication of sheet goods, and trim recovery is important in reducing the costs of sheet goods.
A key property of a polymer useful for thermoforming is its strength in the molten state (“melt strength”). Melt strength helps keep a polymeric material from tearing or excessive deformation when subjected to stress while in the melted state. Conventional polypropylene (“PP”), usually called homopolymer PP (or just “homo-PP”), has poor melt strength and relatively little melt elasticity. Heated to its melting temperature (in a range about 165° C.), it moves from a stiff-rubbery solid to a floppy, syrupy liquid in only a few degrees. In order to make PP more useful for a variety of applications, modifications have been made to it to improve melt strength and melt elasticity. The earliest such modifications focused on copolymerization with ethylenic molecules to produce ethylene-propylene (“EP”) copolymers. However, while EP copolymers have superior sag resistance and good to excellent low-temperature brittleness, they have lower melting temperatures (making them unsuitable for hot environments) and they have less environmental stress crack resistance and cost more than homo-PP. Another early modification to homo-PP to improve its hot strength was the addition of fillers. Talc, calcium carbonate and titanium dioxide at dosages of 10 to 30 wt % improve stiffness at PP melt temperatures, as well as improved room temperature stiffness. But fillers do not usually alter the morphological characteristics of the polymer; and melt temperatures and glass transition temperatures of homo-PP's remain essentially unchanged by adding fillers. For example, if the unfilled homo-PP has a low temperature impact strength that is unacceptable for a given application, the filled homo-PP will also be unacceptable.
Thermoplastic olefins (“TPO's”) are an alternative route to using the characteristics of homo-PP for thermoforming. TPO's are blends of olefin resin and an elastomer in which the olefins are the major continuous phase and elastomer is a minor disperse phase. These blends may be created either by melt blending or by reactor manufacture. TPO's exhibit both thermoplastic and elastomeric properties, i.e., the materials process as thermoplastics but have some physical properties possessed by elastomers.
Molding large parts without surface defects heretofore has required TPO's possessing a very high melt flow rate (“MFR”) or high melt index (“MI”). Accordingly, very high MFR homo-PP is implied as the olefin resin in TPO's for such uses. However, high MFR homo-PP has low melt strength. General purpose TPO blends of homo-PP having a high MFR and olefin elastomer, mixed using either melt processing equipment such as an extruder, or in a reactor, do not have enough melt strength and drawability (extensibility or stretchability) to be shaped into large and deep parts by thermoforming. Addition of inorganic fillers does help to improve melt strengths of TPO's made with very high MFR homo-PP olefins, and such modified compositions can be drawn deeper, but they have much higher densities, making the formed parts too heavy for some applications. Alternatively, high melt strength can be achieved by resorting to TPO compositions made with olefin elastomer and very low MFR (high molecular weight) homo-PP, but these compositions have very low drawability. The inclusion of nucleating agents in TPO formulations to produce highly nucleated PP does facilitate formability but is relatively more expensive and is largely limited to small thin parts, for example, yogurt cups.
Another approach to solving melt and drawability problems of TPO's has been to modify the elastomeric component of the blend. However, using very high molecular weight elastomers or partially or fully cross-linking elastomers can improve melt strength, but at the cost of drawability, increased stiffness, and harder processability.
Thus, the most commercially available thermoplastic olefin solutions are not suitable for large size, deep drawn thermoformed articles. Further, most of the commercially available thermoplastic olefins show excessive shrinkage and loss of gloss after thermoforming. Loss of gloss is a detriment for use on many surface parts as in automobiles and appliances.
It serves understanding of this invention to distinguish thermoplastic olefins (TPO's), sometimes called “hard” thermoplastic compounds, in which the elastomeric component is not the majority component, from thermoplastic elastomers (“TPE's”) and from a subset of TPE's called thermoplastic vulcanizates (“TPV's),” sometimes called “soft” thermoplastic compounds, in which the elastomeric component is the majority component. TPE's are blends of (i) olefin resins or non-olefinic resins, and (ii) olefin or non-olefinic elastomers, in which the resin is the minor component and the elastomer is the major component. Although present as the major constituent, the elastomer is intimately and uniformly dispersed as a discrete particulate phase within a continuous phase of the thermoplastic. In TPE's the elastomer may have been chemically modified (cross linked or “compatibilized”), or may be chemically modified during processing, but not to the extent that the elastomer cannot be processed by itself (that is, there is only a low level of cross-linking).
A TPV is a TPE blend of a thermoplastic resin and a cured elastomer. TPV's may be produced by dynamic vulcanization (sometimes called “DVA's,” for dynamically vulcanized alloys) or by static vulcanization. Early work with vulcanized compositions is found in U.S. Pat. Nos. 3,037,954 and 3,806,558, which disclose static vulcanization as well as the technique of dynamic vulcanization, in which a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer is cured while continuously mixing and shearing the polymer blend. The resulting composition is a microgel dispersion of cured elastomer, such as EPDM rubber, butyl rubber, chlorinated butyl rubber, polybutadiene or polyisoprene in a matrix of thermoplastic polymer such as polypropylene. Static vulcanization is a two step process. The elastomer is first mixed with thermoplastic resin, and in a second step, the elastomer is cured by irradiation or heat or a chemical reaction. Examples of TPE compositions and methods of processing such compositions, including methods of dynamic vulcanization, may be found in U.S. Pat. Nos. 4,130,534; 4,130,535; 4,311,626; 4,594,390; 5,177,147; and 5,290,886. TPE and TPV products are melt processable and can be extruded into profiles such as sheets. They also tend to exhibit high melt strength, but have very little ductility and draw, which reduces the utility of the material technology for processing applications such as thermoforming, blow molding and forming.
The elastomer used as an ingredient in TPE's and TPV's is initially in a gel state and is to be distinguished from an already thermoset elastomer, which is vulcanized to an extent that it cannot be remelted and recycled. To be recycled, a thermoset material (for example, as found in used tires) must be finely comminuted into small particles. For an example of recycled thermoset elastomers used as an ingredient blended into a thermoplastic olefin, see U.S. Pat. No. 6,573,303.
The use of organic peroxide to crosslink and cure the elastomer phase in an olefinic-based TPV is well known to those of ordinary skill in the art. For example, U.S. Pat. No. 3,758,643 discloses that peroxide 2,6-bis(t-butylperoxy)-2,5-dimethylhexane at a concentration of 0.05 to 0.4 weight percent is useful for cross linking the elastomer phase in the olefinic TPV.
Thermoplastics, especially in polymers containing tertiary hydrogen such as polystyrene, polypropylene, polyethylene copolymers etc, but particularly polypropylene, have two types of backbone carbon-hydrogen bonds that can react: secondary and tertiary. Abstraction of hydrogen atoms from these two types of C—H bonds gives rise to secondary and tertiary carbon-centered radicals, respectively. The relevant difference between these two is that it requires substantially more energy to abstract a secondary hydrogen atom than a tertiary hydrogen atom. As a result, secondary radicals are less stable and more reactive than tertiary radicals. Organic peroxides decompose by homolysis of the O—O bond, leaving two oxygen-centered free radicals. These are energetic radicals that tend to be less discriminating in their reactions, meaning that they react more aggressively and with less selectivity, making it more difficult to control the outcome of the reaction to give the highest yield and quality of desired product. It is believed that the more energetic radicals generated from peroxides indiscriminately attack both kinds of backbone hydrogen atoms. It is further believed that β-scission of the polymer backbone (i.e., chain cleavage) occurs preferentially when secondary hydrogen atoms are abstracted, since the resulting secondary radicals are significantly less stable than tertiary radicals. Thus polymers containing secondary and tertiary hydrogen's such as polystyrene, polypropylene, polyethylene copolymers etc, but especially polypropylene, are susceptible to β-scission of the polymer backbone carbon chain (i.e., chain cleavage) due to reaction with highly energetic free radicals generated using peroxides (or from high energy radiation or at elevated temperatures). Chain scission results in lower molecular weights and higher melt flow rates. Because scission is not uniform, molecular weight distribution increases as lower molecular weight polymer chains are formed.
In the context of TPO's (containing cross linkable elastomers), not only does chain scission causes a decrease in the viscosity of the dispersing thermoplastic, but also at the same time the free radicals cause cross linking of the dispersed elastomer, which increases viscosity. Therefore if not very carefully controlled, TPO compositions formed using peroxides as free radical generators have very weak adhesion between the PP and elastomer phases, resulting in poor processability, poor surface finish, poor tear strength, and poor part dimensional stability.
Recently, as exemplified by U.S. Pat. Nos. 6,602,956 and 6,548,600, in the context of TPE's, co-agents have been employed to ameliorate the detrimental effects of peroxide free radical generating agents. In U.S. Pat. No. 6,602,956, the co-agents are metal salts of α,β-unsaturated organic acids, specifically acrylic, methacrylic, maleic, fumaric, ethacrylic, vinyl-acrylic, itaconic, methyl itaconic, aconitic, methyl aconitic, crotonic, alpha-methylcrotonic, cinnamic and 2,4-dihydroxy cinnamic acids, and combinations thereof, or such α,β-unsaturated organic acids in which the pending acid group has been neutralized. In U.S. Pat. No. 6,548,600, the co-agents are monomers or low molecular weight polymers having two or more methacrylate, allyl or vinyl functional groups such as diallyl terephthalate, triallylcyanurate, triallylisocyanurate, 1,2 polybutadiene, divinyl benzene, trimethylolpropane trimethacrylate, polyethylene glycol dimethacrylate, ethylene glycol dimethacrylate, pentaerythritol triacrylate, allyl methacrylate, N N′-m-phenylene bismaleimide, toluene bismaleimide-p-quinone dioxime, nitrobenzene, or diphenylguanidine. The co-agent solution for countering the deleterious effect of peroxide free radical generators operates, as described in U.S. Pat. No. 6,548,600, by the peroxide acting to convert the co-agent into a lower energy state, longer lasting free radical that in turn induces branching in the ethylene elastomer by hydrogen abstraction. Due to the lower energy state of this free radical, β-scissioning and disproportionation of either the polypropylene or ethylene elastomer phase is said to be less likely to occur. Additionally, the coagent is said to have the ability to act as a bridging group between the polymer chains. However, the co-agent solution has its downside. When co-agents are added to control thermoplastic chain scission caused by peroxides or other highly energetic free radicals, the resulting compositions have bad odor, darker color and higher cost.
Other approaches for improving processability or flow in TPE compositions containing polypropylene involve either a reduction in the cure state where the TPE is vulcanized, the use of a polypropylene component having a relatively high MFR, and the addition of high levels of diluent processing oil to the composition. However, while gaining improvement in processability, these solutions have yielded products having reduced tensile strength, elongation, toughness, modulus and heat distortion temperature.
Accordingly, the art has continued to search for a cost effective solution that balances processability and thermo-formability of the thermoplastic composition and mechanical properties of articles formed from the compositions.